Wednesday, 24 November 2010 14:06

Bigeye tuna are among the largest of tuna species and are distinguished as adults by their body depth, colouring (iridescent blue longitudinal band)  and smaller anal and dorsal fins (relative to yellowfin). However, they are more difficult to distinguish from yellowfin tuna as juveniles (~50cm).

In the WCPO, bigeye tuna have a relatively broad distribution, both geographically between 40°N and 40°S,  and vertically between the surface and 500 m deep (occasionally to 1000 m) due to their tolerance of low oxygen levels and low temperatures. In the tropical and subtropical waters or the WCPO, adult bigeye migrate from cooler deeper waters (beneath the thermocline) where they live during the day to shallower warmer waters (above the thermocline) at night.  Juvenile bigeye tend to inhabit shallower waters and can form mixed schools with skipjack and yellowfin, which results in catches by the surface fishery, particularly in association with floating objects.

In the WCPO, smaller bigeye are caught on the surface by a range of gears including handline, ringnet and purse seine and are used mainly for canning, while the majority of larger/older fish are caught by longline fisheries. While bigeye tuna account for a relatively small proportion of the total tuna catch in the region, adult bigeye tuna are extremely valuable (particularly as fresh fish in the Japanese market); their economic value probably exceeds US$1 billion annually.

The typical capture size for bigeye shows two distinct modes in the WCPO, being 20 to 75cm (ringnet, handline, purse seine) which corresponds to fish between 3 months and 1.7 years of age, and  between 100 and 180cm (mostly caught by longline) , corresponding to fish between 2 and 10 years of age. Very few captured fish exceed 200cm or 120 kg.

Bigeye tuna grow more slowly than either yellowfin or skipjack, reaching around 40cm after one year, have a longer lifespan (at least 12 years) and mature later (around 3-4 years of age). Natural mortality is estimated to be relatively low compared with other tropical species. These biological characteristics promote only moderate turnover in bigeye populations, and, in combination with their susceptibility to multiple gear types throughout their lifespan, make bigeye tuna less resilient to exploitation than more productive species like skipjack. The bigeye biomass is estimated to be significantly smaller than those of yellowfin and skipjack in the WCPO.

Like yellowfin, bigeye tuna are highly fecund and can spawn year round over a wide area of the tropical and subtropical Pacific, providing environmental conditions (such as water temperature) are suitable. As with many tropical tuna species, environmental conditions are believed to significantly influence recruitment levels over time.

For stock assessment purposes, bigeye tuna are believed to constitute a single stock in the WCPO.

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